Pot with the wrong value...

Discussion in 'The Projects Forum' started by Calphool, Dec 13, 2008.

  1. Calphool

    Thread Starter New Member

    Dec 13, 2008
    6
    0
    Hi,

    I have access to some cheap motorized potentiometers that I want to use on a project. However, they're of the wrong value. They're 20k motorized pots.

    I need some of 10k value, 1k value, 100k value, 1 mega ohm value, 500k value, and a 5k. If I could gang these motorized pots, I could wire them in parallel to get the lower values (10k, 1k, and 5k)... however, I can't do that. So, I'm thinking I'll wire them with fixed resistors in parallel. Obviously I won't get a linear resistance curve with that, but that's ok, as long as I don't have a mostly right-angle shaped curve. The 10k curve looks fine if I wire a 20k resistor in parallel with the 20k pot. However, the 1k and 5k looks right angled when I do that. Is there some way to get a "more linear" curve?

    Also, is there any way to get those higher valued pots (the 100k, mega ohm, 500k, etc.), maybe using an op amp or something? I'm guessing not, but maybe some smart guy or gal out there has an idea. I'm still kinda learning this hardware stuff (I'm a software guy), so I would appreciate any help anybody could provide.

    Thanks!

    JR
     
  2. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    The best thing is to post some schematics that show what you are doing. It should be possible to use the 20K pots for your purposes, but we need to see what you're doing to figure out if this involves simple or complex changes to your circuits.
     
  3. Calphool

    Thread Starter New Member

    Dec 13, 2008
    6
    0
    Cool. The schematic is here:

    http://www.paia.com/manuals/docs/9308-fatman-manual.pdf

    If you look through the parts list, basically I'm talking about all the pots with an asterisk next to them. I want to replace them with these:

    http://www.goldmine-elec-products.com/prodinfo.asp?number=G9962

    On the 10k ones, I think I can get by just wiring a 20k fixed resistor in parallel to the pot. It's all the rest of them that I'm struggling with what to do. I need to have something at least half way close to a linear resistance curve (otherwise the pot will be too touchy at the elbow of the curve).

    I'm going to drive the motors on the pots with a PIC circuit, but I don't have that circuit put together yet (it's not really material to the discussion anyway, and I think I can figure that part out on my own).

    Any ideas anybody had for those other pots would be greatly appreciated!

    Thanks,

    jr
     
  4. floomdoggle

    Senior Member

    Sep 1, 2008
    217
    2
    No, the required pots in the circuit are required. Paralleing, or in series, will change the parameters of the circuit. Each component is in the circuit to produce the output in a very certain way. Even the manufacturer of each component must be the same. Otherwise, you will not get the same effect. Unlees you are only experimenting, then disregard this post.
    Dan
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    There are a couple of instances where a 20K pot would have no negative effect on circuit operation, but not enough to make comprehensive replacement possible.

    You either need to get matching values or redesign sections of the circuit to work with 20K.
     
  6. Calphool

    Thread Starter New Member

    Dec 13, 2008
    6
    0
    Floomdoggle:

    If this were a digital circuit, maybe I'd agree, but I think there's some room for flexibility here. This is a DIY analog music synthesizer circuit (there are two circuits in the schematic -- a digital one that has nothing to do with any of this, and an analog one that makes the sound).

    According to what I read about resisters, when they're wired in parallel, you get a resistance value using this equation:

    R = 1 / (1/R1 + 1/R2 + 1/R3....1/Rn)

    So, if that's the case, if I take a 20k pot (the motorized one I can get cheaply), and wire it in parallel with a 20k fixed resistor of the appropriate power rating, then I get a resistance curve that looks like this:

    http://www.box.net/shared/qgdl3j6s9n

    The black line represents what the line would look like if I had ganged 2 20k pots together in parallel. However, I can't do that for mechanical reasons. So, the blue line represents what the curve looks like if I just use a fixed 20k resistor instead. That should be tolerable for the 10k pots in the schematic (it won't be perfectly straight, but it should be "reasonable"). However, when I try anything similar with the pots of other values, I get unusable curves (they approach a right angle, which obviously would be very disconcerting if you were trying to use those knobs... they'd do nothing, until they got to the bend in the curve, and then they'd change almost instantaneously to the opposite extreme).

    So, in summary, what I'm hoping, is that someone has some experience in how to get resistance curves similar to the 10k one (maybe with some active circuitry of some kind), but for the other pot values mentioned in the previous post, using the 20k motorized pot as the basis.

    (Either that, or maybe somebody has a sharp idea on how to take a plain old pot of the appropriate value and motorize it myself, while still maintaining the ability to turn the pot from a console... that seems more difficult than trying to figure out some electronic solution, but what do I know?)

    Thanks,

    jr
     
    Last edited: Dec 14, 2008
  7. floomdoggle

    Senior Member

    Sep 1, 2008
    217
    2
    Actually I am talking about analog circuits. I don't know squat about digital, ask anyone here.
    I do have some experience with PAIA products. The kits work better.
    Dan
     
  8. Calphool

    Thread Starter New Member

    Dec 13, 2008
    6
    0
    So I had another idea... although I don't like this one as much, because it introduces digital components into the otherwise purely analog signal path of the Fatman.... but if I can't figure out any other way to do it I might end up with something like this....


    http://www.box.net/shared/zernolmoi5


    This would preserve the motor pots on the control surface, but essentially they'd be "fly by wire" devices.... their 0 - 20k resistance would be translated into a digital value which would in turn be translated into the input to the input to a high res digital pot of the appropriate range (500k, 1k, 1 megaohm, etc.)

    The diagram shows some of the end game here... to use motor pots rather than regular ones, and to update the EEPROM for the 8031 to make the PAIA Fatman respond to Program Change events.... with the control surface literally reconfiguring itself when it receives a Program Change.

    I could probably go pure digital pots, remove all the knobs, and rig up a little PIC based LCD, but that's so uncool. I know it's kind of silly, but I want to preserve the knobs and moving control surface.

    Thoughts?
     
    Last edited: Dec 14, 2008
  9. Calphool

    Thread Starter New Member

    Dec 13, 2008
    6
    0
    I actually am using the kit. I'm modifying it. I want to make it respond to Program Change MIDI events.
     
  10. floomdoggle

    Senior Member

    Sep 1, 2008
    217
    2
    Calphool,
    Then good luck. That you are using the kit would've been helpful in the beginning. So, how are using it?
    Dan
     
  11. Calphool

    Thread Starter New Member

    Dec 13, 2008
    6
    0
    I'm replacing the pots that are on the control surface with motorized pots, and then changing the EEPROM code for the 8031 so that it responds to MIDI Program Change events. The Program Change events will trigger the pots to turn to various combinations of settings... I'll probably also come up with a way to force feed the program change events into the MIDI IN from the control surface of the FatMan. In effect I'm adding Patch Memory to the FatMan, and upgrading its MIDI program to support Patch Changes.The only hang up is that motorized pots are very expensive, and the FatMan uses 18 of them. So, I cannot afford to get perfectly matched replacements, but I have found some very inexpensive 20k motorized pots. So, what I need is some circuit that has two leads on the output side, uses the 20k motor pot, and results in a resistance curve at least somewhat linear, but at different ranges than 20k (500k, 1megaohm, etc.) I don't completely understand the role that each pot is playing in the circuits, so there are probably other ways to achieve the same thing, but from a black box perspective, that's what I need to do.I came up with an idea using digital pots and an analog to digital converter along with the 20k moto pots, but I don't really want to use digital pots in an otherwise purely analog signal path (even though technically it would still be an analog signal path, it still feels like cheating, plus it would be kind of expensive). I can use a fixed 20k resistor in parallel with the 20k moto pot for about 8 of the pots. That leaves another 10 to figure out.Thanks,jr
     
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